In the normal intact knee, the relative position between the femur and the tibia is controlled by the shape of the bearing surfaces and by the 4 major ligaments crossing the knee; the anterior and posterior cruciates and the medial and lateral collaterals. As the knee is flexed and extended, the lengths of the ligaments and the shape of the bearing surfaces work in harmony to maintain stability, without excessive looseness or tightness. When the components of a total knee are inserted at surgery, it is important that the components are inserted accurately, and that the geometry of the components is sufficiently close to anatomic, so that the ligaments can still maintain the correct stability between the femur and the tibia.
In a typical surgical procedure, the bone cuts are made, then the trial components are inserted. At this stage, the knee is flexed and extended to determine whether the knee comes into extension without hyperextending, or having an extension lag. The thickness of the tibial component is adjusted if necessary. Then an assessment is made if the balancing is correct throughout flexion. A simple evaluation method is to remove the trials, introduce a spacer block between the cut surfaces of the femur and tibia with the knee in extension, and move the tibia into varus and valgus to assess the relative stiffnesses. This is repeated at 90 degrees flexion. If either the medial or lateral side of the knee is too tight, the tight ligaments are released.
Methods other than the spacer blocks can be used. One method is to use a distractor tool which inserts between the joint surfaces and equal expansion forces are applied separately to the lateral and medial condyles, equal distraction gaps being an indicator of balancing. More recently, an instrumented tibial trial has been introduced, which takes the place of the tibial trial component, which measures the lateral and medial contact forces throughout flexion and displays the forces on a computer screen.
Thus, there is a need in the art for improved mechanical devices and methods for balancing joints, including knee joints. The present invention meets this need.